1. Field of the Invention
The present invention relates to a device for molding objects, which comprises a mold having a molding space enclosed therein and adapted to be filled by a fluid mold material intended to solidify therein.
2. Description of the Related Art
It is known that it is possible to obtain objects having considerably improved physical properties if the objects are produced by molding and the mold material is allowed to solidify from a molten state under a high pressure. This is, for example, the case in molding objects of different plastics, such as polyethylene, in which the improved physical properties are then obtained by the fact that the high pressure to which the molten plastic is exposed straightens the molecular chains, so that the solidified plastic gets stronger. When molding or casting metals while exposing the molten metal or the metal alloy to high pressures, so-called xe2x80x9cshrinkagesxe2x80x9d in the casting may be pressed out, which substantially increases the strength and the life of the molded object.
It should be noted that the phrase xe2x80x9cfluid mold materialxe2x80x9d used herein is to be given a very broad sense and comprises very thin fluid as well as more viscous, paste-like material.
Typical pressures for molding of this type are 5,000 bars or higher, but this information is not at all to be interpreted as limiting the invention. It is, in practice, nearly impossible to construct a mold for molding a complex object, which can withstand pressures of 5,000 bars or higher for many molding cycles. Therefore, it has been necessary to accept unreasonably high costs should a molding take place under the conditions mentioned above, such that a process for manufacturing the objects in question has not been commercially interesting, particularly when it is desired to produce the same detail a multiple number of times.
The object of the present invention is to provide a device and a method to remedy the inconveniences mentioned above. The invention makes it possible to mold objects while the mold material solidifies under a high pressure in a way that is economically attractive while maintaining molded objects of high quality.
In one embodiment of the present invention such a device is provided, having a chamber adapted to receive the mold during the molding process. The invention also provides a pressure generating means adapted to generate a high pressure such that the pressure exerted on the molding material during the molding is at least the same magnitude as the pressure of the medium filling the chamber. The pressure in the mold and the pressure in the chamber isostatically balances the mold through the molding material and the forces acting upon the medium.
The invention also discloses a method for molding objects in which a fluid mold material under pressure is injected by a supply means to the molding space of a mold and is kept under pressure until the mold material solidifies.
By isostatically balancing the mold, the material of the mold does not have to be particularly resilient even if relatively complex details are molded. The isostatic balancing ensures that the mold may withstand pressures during the solidification of the mold material from a molten state for numerous cycles. Thus, it will be possible to produce molded objects having very good strength properties in comparatively large series without destroying the mold itself.
According to a preferred embodiment of the invention, the device comprises members adapted to register the pressure in the molding space of the mold during the molding, and control the pressure generating means to adjust the pressure of the medium in the chamber depending upon the pressure in the molding space of the mold. In this way, the pressure of the medium in the chamber is always such that the mold is continuously, substantially isostatically balanced independent of pressure changes to which the mold material is subjected in the mold when it solidifies from the molten state.
According to another embodiment of the invention, the mold is formed by a plurality of separable parts, and the pressure generating means is adapted to pressurize the medium in the chamber to a pressure which is slightly higher than the pressure of the molding material in the molding space during the molding, so as to support a sticking of the parts of the mold to each other during the molding. By choosing a somewhat higher pressure of the medium than the pressure to which the mold material is exposed in the mold the medium may in this way be utilized so as to reliably keep the parts of the mold together during the molding process.
According to another embodiment of the invention, a supply means is adapted to supply plastic material in a molten state under high pressure to the molding space. This is particularly advantageous, because molding plastic objects while utilizing the high pressure results in a straightening of the molecular chains within the plastic before solidification occurs. Thus, the result is a substantial improvement in the physical properties of the molded plastic compared with the solidification of such a plastic with no straightening of the molecular chains.
According to another embodiment of the invention, the supply means are adapted to feed the mold material into the molding space of the mold under a pressure exceeding 5,000 bars. Mold material exposed to pressure exceeding 5,000 bars, during the solidification from a molten state exhibits very good physical properties.
According to another embodiment of the invention, the supply means are adapted to feed a metal or metal alloy in its molten state to the molding space of the mold under a high pressure for molding an object. A molding of objects of metal or metal alloys while pressing so-called xe2x80x9cshrinkagesxe2x80x9d out from the casting for obtaining high resistant objects is made possible by this invention.
According to another preferred embodiment of the invention the device comprises an arrangement adapted to control the temperature of the mold material supplied under high pressure to the molding space of the mold as a function of the time the mold material has been located in the molding space. Adapting the very solidification process to the material used, and to the pressures it is exposed to, enables the physical properties of the casting to be optimized.
In a further development of the previous embodiment, the heating arrangement has means for directly or indirectly heating the mold to prolong the time the mold material is present in the molding space before the molding thereof to the object takes place. This may be of a particular advantage should for example a plastic material having molecular chains requiring a comparatively long time for being straightened out be used as mold material. If no such prolongation is necessary for some materials, the heating will then be omitted.